ARM: omap1: Switch to use MULTI_IRQ

This allows us to get a bit further with SPARSE_IRQ and
MULTIARCH support.

Note that we now also rename omap_irq_flags to omap_l2_irq
as that's the omap_irq_flags naming is confusing. It just
contains the interrupt number for the l2 irq.

Cc: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Tony Lindgren <tony@atomide.com>